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Communication Protocols of an Industrial Internet of Things Environment: a Comparative Study

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Communication Protocols of an Industrial Internet of Things Environment: a Comparative Study future internet Article Communication Protocols of an Industrial Internet of Things Environment: A Comparative Study Samer Jaloudi Department of Information and Communication Technology, Al Quds Open University, Nablus 00407, West Bank, Palestine; [email protected] or [email protected]; Tel.: +970-0599-376-492 Received: 4 February 2019; Accepted: 4 March 2019; Published: 7 March 2019 Abstract: Most industrial and SCADA-like (supervisory control and data acquisition) systems use proprietary communication protocols, and hence interoperability is not fulfilled. However, the MODBUS TCP is an open de facto standard, and is used for some automation and telecontrol systems. It is based on a polling mechanism and follows the synchronous request–response pattern, as opposed to the asynchronous publish–subscribe pattern. In this study, polling-based and event-based protocols are investigated to realize an open and interoperable Industrial Internet of Things (IIoT) environment. Many Internet of Things (IoT) protocols are introduced and compared, and the message queuing telemetry transport (MQTT) is chosen as the event-based, publish–subscribe protocol. The study shows that MODBUS defines an optimized message structure in the application layer, which is dedicated to industrial applications. In addition, it shows that an event-oriented IoT protocol complements the MODBUS TCP but cannot replace it. Therefore, two scenarios are proposed to build the IIoT environment. The first scenario is to consider the MODBUS TCP as an IoT protocol, and build the environment using the MODBUS TCP on a standalone basis. The second scenario is to use MQTT in conjunction with the MODBUS TCP. The first scenario is efficient and complies with most industrial applications where the request–response pattern is needed only. If the publish–subscribe pattern is needed, the MQTT in the second scenario complements the MODBUS TCP and eliminates the need for a gateway; however, MQTT lacks interoperability. To maintain a homogeneous message structure for the entire environment, industrial data are organized using the structure of MODBUS messages, formatted in the UTF-8, and then transferred in the payload of an MQTT publish message. The open and interoperable environment can be used for Internet SCADA, Internet-based monitoring, and industrial control systems. Keywords: automation; IIoT; MQTT; MODBUS; publish–subscribe; request–response; SCADA 1. Introduction The Internet of Things (IoT) is an emerging technology that represents a cost-effective, scalable, and reliable ecosystem, proposed for many applications, including smart city sectors [1,2], consumer devices [3,4], industrial environments [5,6], Internet of vehicles [7,8], multimedia [9,10], and 5G systems [11,12]. The IoT platform, from the communications perspective, consists of a TCP/IP network and standard protocols [13]. Standard protocols primarily include advanced message queuing protocol version 1.0 (AMQP 1.0) [14–16], message queuing and telemetry transport (MQTT) [17], constrained application protocol (CoAP) [18], extensible messaging and presence protocol (XMPP) [19], and JavaScript object notation (JSON) [20]. The TCP/IP networks include Wi-Fi [21], Internet, Intranet, and modern mobile networks. The employment of a communication infrastructure and protocol depends on the field of application, the timing requirements, and the data transmission rates [13]. Therefore, the requirements of consumer electronics are different from those of smart city sectors and industrial applications. Future Internet 2019, 11, 66; doi:10.3390/fi11030066 www.mdpi.com/journal/futureinternet Future Internet 2019, 11, 66 2 of 18 In the industry context, Industrial Internet of Things (IIoT) [22,23], Industry 4.0 [24,25], Smart Manufacturing [26,27], and Smart Factory [28,29] are all terminologies for the emerging industrial environments that employ information and communication technologies (ICTs), including IoT platforms, while maintaining industry requirements. The term IIoT refers to the use of IoT technologies in many fields of application such as manufacturing, factories, transportation, gas, oil, and electric grids. However, most industrial and SCADA-like (supervisory control and data acquisition) systems of these fields employ proprietary communication protocols and ICTs, which lead to closed industrial systems. Hence, customers are stuck to a single vendor, costs are high, and interoperability is not fulfilled. In this paper, standard IoT protocols are introduced and compared. Then, MQTT is chosen for machine-to-machine (M2M) communications to complement the MODBUS TCP [30–32] operations in an IIoT environment. This environment integrates an event-based message-oriented protocol, i.e., MQTT, with a polling-based request–response protocol, intended for industrial applications, i.e., MODBUS TCP. The study shows that the MODBUS TCP and MQTT can coexist together, and in parallel, within the same IIoT environment. While industrial requirements of control and monitoring can be met via the MODBUS TCP using the request–response model, the MQTT protocol complements its operation by fulfilling the IoT requirements, using the publish–subscribe pattern for M2M communications. The IoT protocol works in parallel with the MODBUS TCP and relays industrial data to an Internet-based server for remote monitoring, analysis, and archiving. To solve the interoperability problem, industrial data are formatted using the MODBUS message format and transferred in the payload of the MQTT publish messages. In fact, the IoT protocols cannot replace the MODBUS protocol in most industrial applications, especially when an optimized message structure is required in the application layer. A second scenario is proposed here, which is to consider the MODBUS TCP as an IoT protocol, and hence build the IIoT environment with the MODBUS TCP only. This solution is efficient and complies with most industrial applications; however, it only depends on the request–response model. The choice of solution is totally dependent on the requirements of the industrial application. The developed environment can be employed in many industrial applications, including Internet SCADA, Internet-based monitoring, and industrial control systems. Hence, customers are not stuck to a single vendor, costs are reduced, and interoperability is maintained. The remainder of the paper is organized as follows. Section2 presents a related literature survey, Section3 presents a theoretical background of the MODBUS protocol, Section4 compares IoT protocols, and Section5 compares MQTT and the MODBUS TCP. Section6 compares the latencies and resource usage of both protocols. Section7 presents a discussion and Section8 concludes the paper. 2. Related Work In the context of industry, researchers are proposing and examining protocols, networks, and middleware architectures for industrial ICT infrastructure and integration. For integration purposes, the authors of [33] proposed a data-oriented machine-to-machine (M2M) communication middleware based on the ZeroMQ platform for IIoT applications. The researchers of [34] presented a case study for controlling industrial robots and monitoring energy consumption remotely based on Ebbits middleware, which transforms data into web services. Another service-oriented IIoT middleware is proposed in [35] for balancing task allocation between a mobile terminal and a utility cloud service. In [36], a platform based on SystemJ for IIoT is proposed using an FPGA (field programmable gate array), and then tested in an automation system. Reference [37] describes a collaboration-oriented M2M (CoM2M) messaging mechanism for IIoT, based on the platform PicknPack food packaging line. In [38], legacy flexible manufacturing systems are integrated with the SCADA system of an Industry 4.0 framework via Ethernet. However, these research papers investigated middleware architectures for integration purposes. The infrastructure of an IIoT was examined in [39], where the IoT vision in industrial wireless sensor networks was implemented using the IPv6 over low-power wireless personal area network Future Internet 2019, 11, 66 3 of 18 (6LoWPAN) and CoAP. Furthermore, the author of [40] proposed a software-defined network for an IIoT based on new networking technologies. However, these research papers investigated network solutions and architectures. Infrastructure protocols were examined in many studies. For example, the authors of [41] developed an edge IoT gateway to extend the connectivity of MODBUS devices to IoT by storing the scanned data from MODBUS devices locally, and then transferring the changes via an MQTT publisher to MQTT clients via a broker. The researchers of [42] designed and implemented a web-based real-time data monitoring system that uses MODBUS TCP communications, following which all data are displayed in a real-time chart in an Internet browser, which is refreshed at regular intervals using hypertext transfer protocol (HTTP) polling communications. In [43], a MODBUS serial protocol was reported that collects data serially via a RS-232 protocol and transfers the collected data over the ZigBee protocol. In [44], measurements of field devices collected by the MODBUS serial protocol were transferred over HTTP using a Wi-Fi network. However, these papers proposed a gateway as a bridge between MODBUS and the Internet, Intranet, or wireless network. In this study, the MODBUS TCP is proposed to be implemented in two scenarios—either alone, or in parallel with an MQTT publisher
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